Photosensitive resin compositions can have various solubilities in a developer solution when irradiated by light, and photosensitive resin compositions in which solubility in the developer solution is decreased when irradiated by light are referred to as “negative”, whereas, photosensitive resin compositions in which solubility in the developer solution is increased when irradiated by light is referred to as “positive”. Specifically, there are known photosensitive resin composition prepared by dissolving an alkali soluble resin and a quinonediazido group-containing compound as a photoactive compound in an organic solvent, and a so-called chemically amplified photosensitive resin composition prepared by dissolving a base resin having variable alkali solubility due to an action of an acid, and a photoacid generator which generates an acid when exposed in an organic solvent.
Heretofore, photosensitive resin compositions have been used for various purposes, for example, for a so-called permanent film. “Permanent film” is a generic term for a coating film made of a photosensitive resin composition formed on or between parts constituting a product which remains even after the completion of the product.
Specific examples thereof include protective solder resists to be formed on the surface of a circuit board; packaging materials for protection of an element; underfilling materials (sealers) which are formed as a cured resin article in the space between an integrated circuit element and a circuit board, reliability of connection being improved by dispersing stress concentrated in the connection portion into the cured resin article; and those used as a bonding layer for bonding a bonding layer or an integrated circuit element of package parts such as a circuit element with a circuit board (see Patent Documents 1 and 2).
Patent Documents 1 and 2 describe methods for production of electronic parts, which comprises bonding a first substrate on which plural electrodes are formed at intervals with a second substrate on which plural electrodes are similarly formed at intervals at positions corresponding to the above electrodes using a permanent film made of a negative photosensitive resin composition.
Specifically, on the first substrate, a negative photosensitive resin composition is applied so as to coat electrodes, and the portion provided with no electrodes is selectively irradiated with light via a mask, and is then developed.
Consequently, a coating layer on the electrodes is removed to form a bonding layer made of the cured negative photosensitive resin composition between the electrodes formed on the first substrate. The size of the bonding layer in the height direction (size in the thickness direction of substrate) is slightly greater than that of the electrodes formed on the first substrate.
After the surface on which the electrodes and the bonding layer are formed of the first substrate, and the surface on which the electrodes and the bonding layer are formed of the second substrate, are allowed to face each other, they are made to contact.
When subjected to a heat treatment, the bonding layer formed between adjacent electrodes of the first substrate is softened and bonded to the inner wall (side wall of the electrodes and top face of the substrate) in the space between adjacent electrodes of the second substrate. As a result, the first substrate and the second substrate are integrated.
(Patent Document 1) Japanese Patent Application, First Publication No. Hei 6-21149
(Patent Document 2) Japanese Patent No. 2660943
However, according to the methods described in Patent Documents 1 and 2, a space is formed between the bonding layer and the substrate because the composition has poor fluidity, causing problems such as poor adhesion of the bonding layer to the second substrate, and thus sufficient mounting reliability of the connection portion cannot be obtained.